Pressure balancing system for master cylinders of braking systems and operating method thereof

ABSTRACT

A pressure balancing system for a pair of tandem master cylinders. Each tandem master cylinder includes a primary plunger, a secondary plunger, a primary chamber, a secondary chamber and a secondary preload sub-assembly, comprising a secondary preload pin and a secondary preload case. The pressure balancing system comprises a primary balancing system and a secondary balancing system. The primary balancing system includes a primary balancing duct establishing communication of a primary balancing channel with the master cylinder at a region occupied by the secondary plunger when the latter is in an initial rest position, and a primary balancing O-ring forming a primary balancing valve between the primary balancing duct and the primary chamber. The secondary balancing system comprises a secondary balancing duct formed within the secondary preload pin and a second end ending with at least one radial hole, and a secondary balancing O-ring fixed to the secondary preload case.

FIELD

The present invention generally relates to a pressure balancing systemfor tandem master cylinders.

More particularly, the invention relates to a pressure balancing systemof the so-called “stroke controlled” type for a pair of tandem mastercylinders of a braking system of a farm vehicle.

The present invention also relates to a method of balancing controlpressures generated by tandem master cylinders.

BACKGROUND INFORMATION

Use of assemblies consisting of two master cylinders for braking systemsof farm vehicles is widely known. Typically, in farm vehicles, and moreparticularly in tractors, a first and a second pedal, each connected toa respective master cylinder, are indeed used for controlling braking ofa left rear wheel and a right rear wheel, respectively.

A braking system configured in that manner allows operating according toa braking modality known as “steering-by-brake”. According to such abraking modality, when the driver acts on one pedal only, the brakingsystem causes braking on the corresponding rear wheel only, therebyenabling the steering of the farm vehicle. When, on the contrary, thedriver acts on both pedals, the braking system causes braking of bothrear wheels.

Typically, braking systems of farm vehicles are also equipped with abalancing circuit, so that, when both pedals are acted upon, a “balancedbraking” (that is, with substantially the same intensity) is obtained onboth rear wheels.

In order to meet the present regulations, requiring that farm vehiclescapable of travelling at a speed exceeding 40 km/h are equipped with adouble braking circuit, the braking systems for farm vehicles of thekind described above provide moreover for using tandem master cylinders,i.e., master cylinders including two sections each capable of actingupon a respective braking circuit. Thanks to a solution of this kind, abreakage concerning one of the two braking circuits does not affect theoperation of the other braking circuit.

PCT Application No. WO2006103049 describes a braking system including apair of tandem master cylinders. Each one of those master cylindersincludes two pistons defining two sections, of which a first sectioncontrols braking of the front brakes and a second section controlsbraking of the respective rear brake. The system includes a firstbalancing duct, arranged to connect the first sections of the tandemmaster cylinders together, and a second balancing duct, arranged toconnect the second sections of the tandem master cylinders together.Balancing of the pressures generated in both sections of the tandemmaster cylinders is actuated by means of valves connected between thesections of the master cylinders and the respective balancing ducts.Such valves consist of stationary valve seats, formed in the body of thetandem master cylinder, and of sealing gaskets provided in the pistons,which gaskets, by moving together with the pistons, after apredetermined stroke cooperate with the valve seats thereby causing thevalves to open or to close.

The described pressure balancing is a so-called “stroke controlled”balancing, since it is mechanically actuated when the pistons operatingin the master cylinders travel a predetermined initial stroke.

A balancing of this kind has a high reliability, yet it generally givesrise to the problem of an excessive axial size. Actually, the sealinggaskets provided in the pistons must be slidable inside the mastercylinders, in order to enable the balancing ducts to communicate withthe respective sections of the tandem master cylinders over the wholepiston strokes. Such a constraint entails that the lengths of thepistons and the tandem master cylinders are greater than the lengthsrequired in the absence of the balancing function.

For instance, a tandem master cylinder with a stroke controlledbalancing function, like the described cylinder, requires for eachsection a length that, with respect to a tandem cylinder without thebalancing function, is increased by an amount substantially equal to thepiston stroke. Such a length increase is required to enable opening thebalancing valves over the whole piston strokes.

Tandem master cylinders equipped with the stroke controlled balancingfunction have the problem that they have an axial size greater than thatrequired by tandem master cylinders without the stroke controlledbalancing function. Such a problem is of particular relevance especiallyin farm vehicles, where the need to reduce the axial sizes of thedevices installed therein is more and more felt.

Moreover, solutions aimed at reducing the axial size are available inthe field of the braking systems. An example is the hydraulic brakebooster described in European Patent No. EP0200387, which comprises,within a hollow body, a piston having a front and a rear portion, afirst chamber and a second chamber. The brake booster has a duct formedin the hollow body in correspondence of the front piston portion andcommunicating with a tubular duct within the hollow body.

In the field of balancing systems for non-tandem master cylinders, PCTApplication No. WO2012095767 describes use of a balancing duct formedinside a pin located inside the master cylinder and including a valvefor closing the balancing duct, which valve is arranged to be opened bythe stroke of the piston of the master cylinder.

SUMMARY

It is an object of the present invention to provide a pressure balancingsystem for tandem master cylinders, which overcomes the problemsmentioned above of the prior art.

According to the present invention, this object is achieved by means ofa pressure balancing system as described herein.

The following description of the present invention is given in order toprovide a basic understanding of some aspects of the present invention.

This description is not an exhaustive description and, as such, it isnot to be intended as being suitable for identifying key or criticalelements of the present invention, or for defining the object of thepresent invention. It only aims at setting forth some aspects of thepresent invention in simplified form.

In accordance with a feature of the present invention, a pressurebalancing system for tandem master cylinders includes a primarybalancing system including a primary balancing duct ending in the mastercylinder at a region occupied by a secondary plunger when the latter isin rest position, i.e., when a brake pedal associated with the mastercylinder is not being actuated.

In accordance with another feature of the present invention, the primarybalancing system includes a primary balancing O-ring arranged tocooperate in fluid-tight manner with a surface of the secondary plungerso as to form a primary balancing valve between the primary balancingduct and a primary chamber of the master cylinder. Such a valve isclosed when the secondary plunger is in rest position and opens when thesecondary plunger travels a predetermined portion of its stroke, suchthat it no longer cooperates in fluid-tight manner with the primarybalancing O-ring.

In accordance with a further feature of the present invention, thepressure balancing system includes a secondary balancing systemincluding a secondary balancing duct formed within a secondary springpreloading pin and having at least one radial hole at an end thereoflocated near a secondary preload case.

In accordance with yet another feature of the present invention, thesecondary balancing system includes a secondary balancing O-ring fixedlyconnected with the secondary preload case. Such an O-ring cooperates influid-tight manner with the at least one radial hole of the secondarybalancing duct, so as to form a secondary balancing valve between thesecondary balancing duct and a secondary chamber of the master cylinder.The secondary balancing valve is closed when the secondary plunger is inan initial rest position and opens when the secondary plunger travels apredetermined portion of its stroke such that the secondary balancingO-ring does not obstruct the at least one radial hole.

BRIEF DESCRIPTION OF THE DRAWING

The above and other features and advantages of the present inventionwill become apparent from the description of a preferred embodimentsmade by way of non-limiting example with reference to the figure.

FIG. 1 is a longitudinal sectional view of a tandem master cylinder.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Referring to FIG. 1, there is shown, in longitudinal sectional view, atandem master cylinder 10 comprising a pressure balancing systemaccording to the present invention.

Preferably, said tandem master cylinder belongs to an assemblycomprising two tandem master cylinders formed, for instance, in a singlebody and having parallel longitudinal axes.

In accordance with other embodiments, the tandem master cylinders of theassembly could be formed in separate bodies.

The tandem master cylinders of the assembly are substantially identicaland therefore the following description relates to only one of thetandem master cylinders.

A tandem master cylinder 10 equipped with the pressure balancing systemaccording to the present invention includes a hollow body 100, a primaryplunger 101 and a secondary plunger 102, which plungers are slidablymounted in hollow body 100 and define, in hollow body 100, a primarychamber 125 and a secondary chamber 126, respectively, which areseparated from each other in fluid-tight manner.

Plungers 101, 102 and chambers 125, 126 advantageously have acylindrical shape with circular cross-sections developing about alongitudinal axis X-X, whereby they are aligned and mutually coaxial.

Primary and secondary chambers 125, 126 are arranged to contain aprimary fluid and a secondary fluid, respectively, coming from a primarysupply circuit and a secondary supply circuit, respectively.

In accordance with the present invention, the primary and secondarysupply circuits, supplying the primary chamber of each of the two tandemmaster cylinders of the assembly and the secondary chamber of each ofthe two tandem master cylinders of the assembly, respectively, areindependent from each other, so as to ensure braking in case of breakageof one of the supply circuits.

The primary and secondary fluids within primary and secondary chambers125, 126 have a primary and a secondary control pressure, respectively,which depends on an actuating force applied to a pedal associated withthe tandem master cylinder.

The balancing system of each tandem master cylinder of the assemblycomprises a primary balancing system and a secondary balancing system.

More particularly, the primary balancing system is arranged to obtainbalancing between the primary control pressure present in the primarychamber of one tandem master cylinder and the corresponding primarycontrol pressure in the other tandem master cylinder of the assembly.

The secondary balancing system is instead arranged to perform balancingbetween the secondary control pressure present in the secondary chamberof one tandem master cylinder and the corresponding secondary controlpressure of the other tandem master cylinder of the assembly.

Primary plunger 101 of tandem master cylinder 10 is connected in aconventional manner with a push rod (not shown) in turn connected to thepedal (not shown), and is kept spaced from secondary plunger 102preferably by means of a primary spring 106. In turn, secondary plunger102 is kept spaced from a bottom 135 of hollow body 100 preferably bymeans of a secondary spring 107.

Primary plunger 101 is associated with the respective push rod in aconventional manner, for instance through a rounded hollow 123, forinstance a hollow with concave spherical shape, which is formed at afirst end 101 a of primary plunger 101 facing the outside of hollow body100 and is arranged to substantially fit a head (not shown) of the pushrod.

The primary spring keeping primary plunger 101 spaced from secondaryplunger 102 is mounted between a second end 101 b of primary plunger101, facing primary chamber 125, and a first end 102 a of secondaryplunger 102, also facing primary chamber 125.

Primary spring 106 is a spring preloaded by means of a primary preloadsub-assembly, which preferably includes a primary preload case 108 and aprimary preload pin 109.

Primary preload case 108 is a hollow case having a base kept against thesecond end 101 b of primary plunger 101 by primary spring 106, and ahead, in distal position relative to the base, having a hole 129.

Primary preload pin 109 has a first end fixedly secured, e.g., screwed,to the first end 102 a of secondary plunger 102 and, at the other end, awidened head 127 located inside primary preload case 108.

Primary preload pin 109 and primary preload case 108 are arranged tocooperate with each other by sliding one inside the other through hole129, so as to take a plurality of positions between an initial restposition (shown in the Figure), in which widened head 127 abuts againstthe head of case 108, and an end position, in which widened head 127 isclose to the second end 101 b of primary plunger 101.

In the initial rest position mentioned above, the second end 101 b ofprimary plunger 101 and the first end 102 a of secondary plunger 102 areat a maximum distance, determined by primary preload case 108 andprimary preload pin 109. Since such a distance is shorter than thelength of primary spring 106 at rest, the latter is consequently in apreloaded condition.

The primary preload sub-assembly preferably further includes a locatingcup 110, abutting against the first end 102 a of secondary plunger 102and intended to receive the head of primary preload case 108 when case108 and pin 109 are in the end position mentioned above.

The elements forming the primary preload sub-assembly preferably havecylindrical symmetry about longitudinal axis X-X.

Preferably, an atmospheric O-ring 105 and a primary O-ring 103 a arehoused within suitable circumferential recesses formed in hollow body100 and facing the inside thereof.

More particularly, atmospheric O-ring 105 is located between anon-closed end of hollow body 100 and a primary supply duct 118 radiallyformed in hollow body 100.

Primary supply duct 118 is arranged to establish communication between aprimary supply circuit and the inside of hollow body 100, in particularprimary chamber 125, and it preferably ends in a circumferential recess131 provided in hollow body 100 and facing the inside thereof.

Atmospheric O-ring 105 is arranged to cooperate with the surface ofprimary plunger 101 sliding in contact with hollow body 100, so as toensure tightness between circumferential recess 131, and hence primarysupply duct 118, and the outside of tandem master cylinder 10, whichgenerally is at atmospheric pressure.

Primary O-ring 103 a is located between primary supply duct 118 andprimary chamber 125 and is arranged to cooperate with the surface ofprimary plunger 101 sliding in contact with hollow body 100, so as tobuild a cutoff valve, of a kind known per se, between primary supplyduct 118 and primary chamber 125. More particularly, primary O-ring 103a allows connecting primary supply duct 118 to primary chamber 125,through an opening 133 in primary plunger 101, when primary plunger 101is in the initial rest position mentioned above and shown in the Figure,and cuts off such a connection, by cooperating with primary plunger 101,when the latter starts moving inside hollow body 100 due to theactuation of the respective pedal.

In accordance with the present invention, the primary balancing systemof tandem master cylinder 10 includes a primary balancing duct 115 and aprimary balancing valve. More particularly, primary balancing duct 115is formed in hollow body 100 of tandem master cylinder 10 and isconfigured for establishing communication between primary chamber 125 oftandem master cylinder 10 and a primary balancing channel (not shown inthe Figure), in turn communicating with a respective primary balancingduct and a primary chamber of the second tandem master cylinder of theassembly.

More particularly, primary balancing duct 115 ends in hollow body 100 ina circumferential recess 128, provided in hollow body 100 and facing theinside thereof, which is located at a region occupied by secondaryplunger 102 when the latter is in an initial rest position, shown in theFigure.

Preferably, circumferential recess 128 where primary balancing duct 115ends is located between two further circumferential recesses in hollowbody 100, in which a primary balancing O-ring 116 a and an O-ring 116 bare housed. Such O-rings 116 a, 116 b are configured so that, bycooperating with the surface of secondary plunger 102 sliding in contactwith hollow body 100, they also ensure tightness between primarybalancing duct 115 and primary chamber 125 and between primary balancingduct 115 and a secondary balancing duct 119, respectively: The laterduct will be described in more detail below.

Preferably, O-ring 116 b can have a toroidal diameter greater thanO-ring 116 a and can be housed in a circumferential recess having awider portion on its side proximal to primary balancing duct 115 and anarrower portion on the other side. Such a constructional detail allowsO-ring 116 b to act as an absorber, i.e., in case of a pressure increasein primary balancing duct 115, the O-ring is compressed towards thenarrower portion, thereby making available, in the circumferentialrecess, space for the pressurised fluid in primary balancing duct 115and limiting in this manner the pressure increase.

Primary balancing O-ring 116 a and secondary plunger 102 form togetherthe primary balancing valve, which allows connecting primary balancingduct 115 and primary chamber 125 when, upon actuation of the pedal,secondary plunger 102 travels a predetermined portion of its strokeinside hollow body 100 such that it no longer cooperates in fluid-tightmanner with primary balancing O-ring 116 a.

Moreover, when primary plunger 101 continues its stroke up to primarybalancing O-ring 116 a, a longitudinal groove 120 provided in primaryplunger 101, between primary plunger 101 and hollow body 100, allows notclosing the communication between primary balancing duct 115 and primarychamber 125, at least until primary plunger 101 arrives close to the endof its stroke, in which region balancing is no longer useful, since thetandem system compensates itself.

Secondary spring 107 keeping secondary plunger 102 spaced from bottom135 of hollow body 100 is mounted between said bottom 135 and a secondend 102 b of secondary plunger 102, facing secondary chamber 126.

Secondary spring 107 is a spring preloaded by a secondary preloadsub-assembly, which preferably includes a secondary preload case 111 anda secondary preload pin 114, the case and the pin preferably havingcylindrical symmetry about longitudinal axis X-X.

Secondary preload case 111 is a hollow case having a base kept againstthe second end 102 b of secondary plunger 102 by secondary spring 107,and a head, in distal position relative to the base, having a hole 130.

Secondary preload pin 114 has a first end fixedly secured, e.g., througha screwing terminal 121, to bottom 135 of hollow body 100, and a head,located inside secondary preload case 111, on which a stop ring (alsoknown as circlip or Seeger ring) 113 is mounted.

Preferably, a containment case 138 having the shape of a hollowcylinder, coaxial with longitudinal axis X-X, is fitted, for instancewith clearance, within secondary preload case 111, near the headthereof. Such a containment case 138, the function of which will beexplained later on, is held in position, in the axial direction, on theone side by means of the head of secondary preload case 111 and on theother side by means for instance of suitable lugs 122 formed on aninternal surface of secondary preload case 111.

Secondary preload pin 114 is arranged to cooperate with secondarypreload case 111 and containment case 138 by sliding inside them throughhole 130, so as to take a plurality of positions between an initial restposition (shown in the Figure), in which stop ring 113 abuts againstcontainment case 138, in turn abutting against the head of secondarypreload case 111, and an end position, in which the head of secondarypreload pin 114 is close to the second end 102 b of secondary plunger102.

In the initial rest position mentioned above, the second end 102 b ofsecondary plunger 102 and bottom 135 of hollow body 100 are at a maximummutual distance, determined by secondary preload case 111, containmentcase 138 and secondary preload pin 114. Since such a distance is shorterthan the length of secondary spring 107 at rest, the latter isconsequently in preloaded condition.

Tandem master cylinder 10 further includes a secondary balancing system,including a secondary balancing duct 136 and a secondary balancingvalve. Secondary balancing duct 136 is formed within secondary preloadpin 114 and has a first end, near bottom 135 of hollow body 100,connected to a secondary balancing channel 117, in turn communicatingwith a respective secondary balancing duct and a secondary chamber ofthe second tandem master cylinder of the assembly. A second end ofsecondary balancing duct 136, located close to the head of secondarypreload pin 114 and hence in distal position relative to bottom 135 ofhollow body 100, ends with at least one radial hole 137, e.g. with tworadial holes.

In accordance with the present invention, a secondary balancing O-ring112, facing the inside of containment case 138, is fitted in containmentcase 138 and is arranged to cooperate, in fluid-tight manner, withsecondary preload pin 114, and more particularly with radial holes 137of secondary balancing duct 136, so as to form the secondary balancingvalve, which allows establishing communication between secondarybalancing duct 136 and secondary chamber 126. More particularly, saidvalve is closed when secondary balancing O-ring 112 obstructs radialholes 137, i.e. when secondary plunger 102 is in its initial restposition mentioned above, and opens when the secondary plunger hastravelled a predetermined portion of its stroke, due to the actuation ofthe pedal, such that secondary balancing O-ring 112 no longer obstructsradial holes 137.

Preferably, moreover, a secondary supply duct 119 is radially formed inhollow body 100 and is arranged to establish communication between asecondary supply circuit and the inside of hollow body 100, inparticular secondary chamber 126, by preferably ending in acircumferential recess 132 provided in hollow body 100 and facing theinside thereof. Preferably, circumferential recess 132 is located in anintermediate position between bottom 135 of hollow body 100 and primarybalancing duct 115

Preferably, a first secondary O-ring 103 a is housed within a suitablecircumferential recess formed in hollow body 100 and facing the insidethereof, between secondary supply duct 119 and secondary chamber 126.Such a first secondary O-ring 103 b is arranged to cooperate with thesurface of secondary plunger 102 sliding in contact with hollow body100, so as to build a cutoff valve, of a conventional kind. That valveallows connecting secondary supply duct 119 and secondary chamber 126,through an opening 134 in secondary plunger 102, when secondary plunger101 is in the initial rest position mentioned above and shown in theFigure, and cuts off such a connection when secondary plunger 102 startsmoving inside hollow body 100 due to the actuation of the pedal.

Tandem master cylinder 10 preferably further includes a secondary O-ring104 located in a suitable circumferential recess provided in hollow body100 and facing the inside thereof. Secondary O-ring 104 is arranged tocooperate in fluid-tight manner with the surface of secondary plunger102 sliding in contact with hollow body 100, so as to ensure tightnessbetween circumferential recess 132, and hence secondary supply duct 119,and primary supply duct 115.

In accordance with a variant of the preferred embodiment, O-ring 116 bcould possibly be dispensed with and its function could be performed byO-ring 104 disclosed above.

Primary chambers of both tandem master cylinders of the assembly arehydraulically connected to a respective primary utilising device, forinstance a right rear brake and a left rear brake of a farm vehicle orthe like.

Similarly, secondary chambers of the tandem master cylinders of theassembly are hydraulically connected to a respective secondary utilisingdevice, for instance a right front brake and a left front brake of thefarm vehicle or the like.

Each one of the primary and secondary chambers is connected to therespective utilising device in known manner, through a primary outletopening and a secondary outlet opening (not shown), respectively, formedin hollow body 100.

Hereinafter, the method of balancing the primary control pressurespresent in the primary chambers of both tandem master cylinders of theassembly and the secondary control pressures present in the secondarychambers of both tandem master cylinders of the assembly will bedisclosed in different situations of use and in case of malfunctions.

In case a single pedal, and hence a single tandem master cylinder, isactuated, initially the resistance offered by secondary spring 107,which preferably is lower than that of primary spring 106, is overcome.At the same time, through the cooperation of the push rod, primaryplunger 101 and primary spring 106, plungers 101 and 102 of tandemmaster cylinder 10 simultaneously move, thanks to the lower resistanceoffered by secondary spring 107 in comparison to primary spring 106,thereby opening the balancing valves of the tandem master cylinder beingactuated. When a single tandem master cylinder of the assembly isactuated, since the primary and secondary balancing valves of the tandemmaster cylinder that is not being actuated remain closed, no fluidpassage occurs between primary chambers 125 and secondary chambers 126of the tandem master cylinders through balancing ducts 115 and 136 andthe balancing channels.

Subsequently, a further displacement of plungers 101 and 102 of tandemmaster cylinder 10 causes simultaneous closure, apart fromconstructional tolerances, of the fluid communication between supplyducts 118, 119 and the primary and secondary chambers 125 and 126,respectively, thereby generating primary and secondary controlpressures, respectively, in said chambers.

If a malfunction in primary control pressure delivery towards theassociated primary utilising device occurs in primary chamber 125,primary plunger 101 compresses primary spring 106 by abutting againstthe secondary plunger and pushing it. Thus, a secondary control pressureis anyway generated in secondary chamber 126 and will be transmitted tothe secondary utilising device. This feature is particularly appreciatedin a braking system for farm vehicles. Indeed, if pressure delivery tothe primary utilising device, e.g. the right or the left rear brake ofthe farm vehicle, has a malfunction, at least the secondary utilisingdevice, for instance the front brake on the same side as themalfunctioning rear brake, can intervene.

Similarly, if a malfunction in secondary control pressure deliverytowards the associated secondary utilising device occurs in secondarychamber 126, secondary plunger 102 compresses secondary spring 107 byabutting against bottom 135 of hollow body 100. Yet, this does notprevent generation of a primary control pressure in primary chamber 126,which will be transmitted to the primary utilising device.

If the pedals associated with both tandem master cylinders of theassembly are actuated, the primary and secondary plungers of both mastercylinders generate a control pressure in the respective primary andsecondary chambers. Moreover, all balancing valves open and, throughsuch valves, fluid passes between the primary chambers of both tandemmaster cylinders, through primary balancing ducts 115 of the respectivetandem master cylinders and the primary balancing channel, as well asbetween the secondary chambers of both tandem master cylinders, throughsecondary balancing ducts 136 of the respective tandem master cylindersand the secondary balancing channel. In this manner, a balancing betweenthe primary control pressures generated in the primary chambers and thesecondary control pressures generated in the secondary chambers isjointly achieved.

When a malfunction in primary control pressure delivery towards theassociated primary utilising device occurs in a primary chamber of oneof the tandem master cylinders, primary plunger 101 compresses primaryspring 106 and pushes secondary plunger 102 with a consequent opening ofthe primary balancing valve, which remains open thanks to the provisionof longitudinal groove 120. In this manner, a balancing between theprimary control pressures supplied by the primary chambers of bothtandem master cylinders is obtained, and the lengthening of the strokeof the malfunctioning primary plunger is moreover limited.

This balancing method is particularly appreciated in a braking systemfor farm vehicles. Indeed, let us assume that a drop occurs in theprimary control pressure taken by the fluid contained in one of theprimary chambers and directed, for instance, to one of the rear brakes.In such case, the fact that the balancing of primary control pressuresoutgoing from the primary cambers is anyway made possible allows in anycase balanced actuation of the rear brakes.

Similarly, if a malfunction in secondary control pressure deliverytowards the associated secondary utilising device occurs in a secondarychamber of one of the tandem master cylinders, a similar advantageoussituation occurs in which the primary control pressure generated in theprimary chamber pushes the secondary plunger, with a consequent openingof the secondary balancing valve. In this manner, a balancing of thesecondary control pressures is achieved.

Of course, obvious modifications and/or changes to the above descriptionare possible, in respect of the sizes, the shapes, the components, theconnections and the contacts, of the assembly of the tandem mastercylinders disclosed and in the method of balancing them without therebydeparting from the present invention.

For instance, even though the preceding description refers to tandemmaster cylinders without a servo-control function, the cylinders couldbe equipped with a servo-control function, as it is well known to theskilled in the art, for instance by means of a brake booster withparallel axes of the “single block” type, or by means of two separatebrake boosters, one for each tandem master cylinder.

1-9. (canceled)
 10. A pressure balancing system for a pair of tandem master cylinders, each of the tandem master cylinders including, within a hollow body, a primary plunger, a secondary plunger, a primary chamber between the primary plunger and the secondary plunger, a secondary chamber between the secondary plunger and a bottom of the hollow body, and a secondary preload sub-assembly arranged within said secondary chamber, the secondary preload sub-assembly including a secondary preload pin and a secondary preload case, the pressure balancing system comprising, within each of said tandem master cylinders: a primary balancing system, including: a primary balancing duct provided in the hollow body of the tandem master cylinder, which primary balancing duct communicates a primary balancing channel located between the pair of tandem master cylinders with the inside of the hollow body at a region occupied by the secondary plunger when said secondary plunger is in an initial rest position, and a primary balancing o-ring arranged for forming a primary balancing valve between the primary balancing duct and the primary chamber, the primary balancing valve being closed when the secondary plunger is in the initial rest position, and being arranged for opening when the secondary plunger travels a predetermined portion of its stroke; and a secondary balancing system, including: a secondary balancing duct provided within the secondary preload pin and having a first end connected to a secondary balancing duct located between the pair of tandem master cylinders and a second end ending with at least one radial hole, and a secondary balancing o-ring which is fixed to the secondary preload case and is arranged for forming, by cooperating in a tight manner with the at least one radial hole of the secondary balancing duct, a secondary balancing valve between the secondary balancing duct and the secondary chamber, the secondary balancing valve being closed when the secondary plunger is in the initial rest position and being arranged for opening when the secondary plunger travels a predetermined portion of its stroke such that the secondary balancing o-ring does not obstruct the at least one radial hole any longer.
 11. The pressure balancing system according to claim 10, wherein said secondary balancing o-ring is fitted in a containment case mounted within the secondary preload case.
 12. The pressure balancing system according to claim 11, wherein the containment case is held in position, along the axial direction, by specific lugs formed on an internal surface of the secondary preload case.
 13. The pressure balancing system according to claim 10, wherein the primary balancing o-ring is housed in a circumferential recess provided in the hollow body and forms the primary balancing valve by cooperating in a tight manner with a surface of the secondary plunger which slides in contact with the hollow body.
 14. The pressure balancing system according to claim 10, wherein an o-ring housed in a circumferential recess provided in the hollow body is arranged for cooperating in a tight manner with a surface of the secondary plunger so as to assure a sealing contact between the primary balancing duct and a secondary supply duct of the secondary chamber.
 15. The pressure balancing system according to claim 10, wherein a longitudinal groove is provided in the primary plunger, between the primary plunger and the hollow body, and, when the primary plunger continues its stroke up to the primary balancing o-ring, is arranged for not closing the communication between the primary balancing duct and the primary chamber, at least until the primary plunger is close to an end of its stroke.
 16. A method for operating a pressure balancing system for a pair of tandem master cylinders, each of said tandem master cylinders including, within a hollow body, a primary plunger, a secondary plunger, a primary chamber, and a secondary chamber, the method comprising: actuating a pedal of each of the tandem master cylinder; sliding the primary plunger and secondary plunger of each of the tandem master cylinders; opening, in each of said tandem master cylinders, a primary balancing valve between the primary chamber and a primary balancing duct; opening, in each of said tandem master cylinders, a secondary balancing valve between the secondary chamber and a secondary balancing duct provided within a respective secondary preload pin , so that a secondary balancing o-ring, fixed to a respective secondary preload case, does not cooperate in a tight manner with the secondary balancing duct any longer, by virtue of the sliding of the secondary plunger and, therefore, of the secondary preload case fixed to the secondary plunger, mutually balancing, in a stroke-controlled manner, primary control pressures generated in the primary chambers and mutually balancing secondary control pressures generated in the secondary chambers, by opening the primary and secondary balancing valve, respectively, and holding the primary balancing valve open, in each of said tandem master cylinders, when the primary plunger continues its stroke up to primary balancing o-ring, by means of a longitudinal groove provided in the primary plunger.
 17. The method according to claim 16, wherein the step of opening a primary balancing valve provides for sliding the secondary plunger so that said secondary plunger does not cooperate in a tight manner with the primary balancing o-ring any longer. 